In Lake Biwa, there are two peaks of 2-methylisoborneol (MIB) production, each of which is due to the different MIB producing algae, Phormidium and Oscillatoria. The former grows in early summer, whereas the latter grows in late summer. Levels of MIB were determined for water samples collected from Lake Biwa and the lower sampling site of Murano Water Treatment Plant's intake on Yodo River. MIB concentrations detected at the lower site differed between the two odor seasons. In the first episode due to growth of Phormidium, MIB level at the lower site was almost the same as that in the lake, whereas it was clearly lower than that of the lake in the second episode caused by Oscillatoria. No differences in flow ratio of the lake effluent to that of Yodo River were detected between the two odor seasons. Thus, the difference in MIB concentration between the lake and the lower site in the second episode may be caused by a decrease in cell-associated MIB due to the high sinking rate of Oscillatoria. The sinking rate ofOscillatoria trichome was twenty times higher than that of Phormidium.

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